Non-silver imaging compositions relying upon the conversion of cobalt(III) complexes to cobalt(II) and released ligands are described in a number of publications, for example, Research Disclosure, Vol. 184, Publication No. 18436 dated August 1979, published by Industrial Opportunities Ltd., Homewell, Havant, Hampshire, PO9 1EF United Kingdom. In one form, e.g., Examples 8 and 9 thereof, a quinone photoreductant and o-phthalaldehyde, hereinafter "phthalaldehyde," are included, in one or more layers, with the cobalt(III) complex. Upon exposure to light, the photoreductant forms a reducing agent for the complex. Upon development by heat, the ligands of the complex are released to produce, with the phthalaldehyde, a black dye. A variation of this composition is also described in Research Disclosure, Publication No. 18436, Vol. 184, August, 1979, wherein photoinhibitors are added to permit light radiation to imagewise inhibit the composition from forming a dye.
Such imaging compositions have been found to be highly useful, particularly for contact positive or negative imaging, when coated on a support as an imaging element. However, the thermal development of the image frequently requires temperatures that exceed 125.degree. C., and sometimes even 135.degree. C. It has been found that invariably, such high temperatures cause significant dimensional changes in the afore-described imaging element. In some cases, these changes tend to interfere with proper image registration. Specifically, the high thermal development temperatures cause dimensional changes in either the width and/or length, that exceed .+-.0.03%. Such changes prevent accurate color registration when using color separation negatives or positives prepared from these imaging compositions.
Prior to this invention, it has been recognized that imaging elements comprising, a light-sensitive silver halide layer on a poly(ethylene terephthalate) support can be adversely affected by thermally-induced dimensional changes incurred by the support alone. Various heat-relaxing techniques have been applied to the support prior to the coating of the light-sensitive silver halide layer(s), to eliminate or reduce that aspect of the problem. Examples are described in U.S. Pat. No. 2,779,684, and British Pat. No. 1,000,361, published Aug. 4, 1965. However, even when those specially treated supports are used in the imaging elements noted above, the dimensional changes that occur upon thermal development are still unacceptable, that is, they still exceed .+-.0.03% in width or length.
U.S. Pat. No. 3,939,000, issued on Feb. 17, 1976, suggests that an alternative to heat-relaxing a poly(ethylene terephthalate) support alone is to heat-treat the fully-coated element prior to use, at least in those cases where the light-sensitive layer is silver halide. The purpose is to eliminate curl such as occurs during roll storage. Such a purpose differs, of course, from the concern arising from dimensional changes caused by thermal development of an image. However, to determine whether the process described in the U.S. Pat. No. 3,939,000 also reduces dimensional changes as would occur upon thermal development at 125.degree. C. for five seconds, a silver halide element comparable to that described in the U.S. Pat. No. 3,939,000 was tested. It has been found that heating such silver halide elements in the manner indicated by Example 1 of U.S. Pat. No. 3,939,000 is not successful in reducing dimensional changes to no greater than .+-.0.03% in either width or length.